High frequency amplifier circuits



i 1943., M. J. o. STRUTT ETAL 2,317,824

HIGH FREQUENCY AMPLIFIER CIRCUIT Filed Jan. 29,. 1941 2 Sheets-Sheet l MJOX$$W ZwL Mum,

ATTORNEY M. J. o. STRUTT ETAL HIGH FREQUENCYL AMPLIFIER CIRCUIT Aim 27, 1943.

2 Sheets-Sheet 2 Filed Jan. 29, 1941 ATTORNEY Patented Apr. 27, 1943 HIGH FREQUENCY AMPLIFIER CIRCUITS Maximiliaan Julius Otto Strutt and Alder-t van der Ziel, Eindhoven, Netherlands; vested in the Alien Property Custodian Application January 29, 1941, Serial No. 376,456 In the Netherlands March 26, 1940 12 Claims.

In our copending application, Ser, No. 333,088, filed May 3, '1940, there isdescribed a high-frequency' amplifier circuit comprising at least one amplifier valve whose cathode is provided with at least two leads, one of which, for the oscillations to be amplified, is included in "the input circuit and another in the output circuit, and in which capacities are connected between the control grid and the point of the cathode which is connected to the cathodelead included in the input circuit and between the control grid and that end of the cathode lead included in the output circuit which is not connectedto the cathode, and one or both cathode leads comprises or com prise impedances of which at least one has an inductive character, said capacities and impedances being proportioned in such manner that the'input damping is approximately zero or negative.

The invention'relates to a cascade circuit comprising at least two'amplifying stages built up in the manner'abovedescribed.

According to the invention, in a cascade circuit of the said kind, at least at the stages preceding the last stage, that end of the cathode lead included in the output circuit which is remote from the cathode is connected to earth for highfrequency currents.

In order that the invention may be clearly understood and readily carried into efiect it will now be described more fully with reference to the accompanying drawings, in which 'Fig. 1 shows a circuit according application, 1 I

Figs. 2, 3 and 4 show cascade circuits according to the present invention.

Fig. 4a, shows a modification of a portion of the circuit disclosed in Fig, 4.

The circuit shown in Fig. 1 comprises an amplifier valve I having a cathode 2 which is provided with two leads, a control grid 3, a screening grid 4. a suppressor grid 5 and an anode I. For the sake of simplicity Fig. 1 only shows the high-frequency connections of the various electrodes, the direct-current connections being omitted.

The high frequencies to be amplified are fed to an input oscillatory circuit 12 which is connected on the one hand to the control grid 3 and on the other hand, via one-of the cathode leads, to the cathode 2. cathode 2 via an output-oscillatory circuit l3, from which the amplified oscillations are taken oil, and via the other cathode lead.

The cathode leads include inductances L1 and L2 and the point 8 of the inductance L1, which point is adjacent to the cathode, is connected to the control grid 3 via a capacity C1 and the end of the inductance L: (9) which is remote from the cathode is connected to the control grid via a capacity C2.

The screening grid 4 is connected, via 2;. capacity C3, to that endof the inductance L2 which is remote from the cathode so that the natural capacity between the control grid and the screen ing grid assists in the action of the capacity C2. The suppressor grid 5' is also connected to the point 9.

If the elements L1, L2, C1 and C2 of the circuit described so far are correctly proportioned this .circuit permits of the input damping, which occurs due to the finite transit time of'the electrons, being suppressed entirely so that the conto the earlier 'sequential input damping becomes zero or even negative. The elements L1, L2, C1 and C2 can be formed in part but not all of them by the natural inductances of the cathode leads and the natural tube capacities In practice the circuit arrangement is fitted on the metal mounting plate of large surface (chassis) and the circuit must be earthed at at least one point for high-frequency currents, that is to say, be connected to the mounting plate,

In th usual high-frequency amplifier circuit arrangements both that end of the input oscillatory circuit H which is remote from the control grid and that end of the output oscillatory circuit l3 which is remote from the anode are frequently connected for high-frequency currents to the chassis. Such double earthing is, however, inadmissible in the present case because the ends 9 and 2|, remote from the cathode, of the inductances L1 and L2 would thus be short-circuited and hence the two cathode leads would be united to form a. single unit. In the circuit arrangement shown in Fig. 1 only one point should therefore be connected to earth for high-frequency currents.

The anode l is connected to the 1 Since theparts of the two cathode leads which are located within the tube form part of the inductances L1 and Lathe cathode 2 is inaccessible. We therefore only have the choice between earthing a point of the input circuit and earthing a point of the output circuit.

In this it should b taken into account that that end of the output oscillatory circuit l3 which is connected to the anode will generally manifest a rather substantial parasitical capacity relatively to the chassis, which is denoted by 20 in Fig. 1. Now,-if 'a point of the input circuit, say the point 2], is earthed, part of the anode current will flow to earth via the, capacity 20 and thence pass to the cathode 2 via the inductancev L1. This causes an undesired retroaction which also brings about a damping of the input-oscillatory circuit. Since the capacity 20 forms part of the output-oscillatory circuit the current passing through this capacity may be larger under certain circumstances than the total anode cursistance 2!, 2B and 21,21.

rent and the desired reduction of the damping of the input-oscillatory circuit will not therefore be obtained by this phenomenon.

Earthing a point of the input circuit is there.

' Iore only permissible if the capacity 20 is reduced to a minimum value, as may be efiected by capacitatively screening the accessible part of the anode lead and the output oscillatory circuit i3 by means of a screen which is connected to the pointO. i

If a pointer the output circuit is connected to earth the phenomenon described cannot occur.

and the said capacitative screening of the output circuit is consequently redundant. In this case the earth connection will preferably be arranged at the point 9 so that the total anode current.

.passes through the inductance La, and the tuning condenser oi the circuit i3 is also earthed.

As is apparent from the above, in a circuit comprising a single amplifying stage both earthing of the point 21 and earthing of the point 9 are possible. This, however, is no longer the case in a cascade circuit.

If, as 'a matter of fact, the amplifying" stage shown in Fig. 1 is followed by a second similar stage the output oscillatory circuit i3 is coupled anti-phase with the anode current of the second stage the decrease of the input damping oi-the second stage which is aimed at by the invention is assisted by the anode current or the first stage.

The circuit shown in Fig. 2 may be enlarged with any number of stages. If the second stage is followed by a third stage the anode If is connected through the condenser 22" to the control grid or the third stage and that end of the oscillatorycircuit l3 which is remote from the anode 'i' through the condenser 28' to that end of the cathode lead included in the input circuit which is remote from the cathode. It the second stage is also the last stage thecircuit I! may be connected to earth through the condenser 28'.

Fig. 3 shows a circuit which can ohly be used 7 it the second stage is also the last stage. The

first stage of the circuit is substantially similar to the first stage of the circuitshown inFig. 2

except that the outputoscillatory circuit II is connected. to earth. To'avoid ashort-circuit oi the source of anode voltage the circuit II includes a blocking condenser 29.

in some manner to the control grid of the next tube and this control grid always has a comparatively considerable capacity relatively to earth. In this case the capacity 20 can therefore-no longer be removed by a screen.

- The circuit is also constitutes the input oscillatory circuit 01' the second stage so that the .input circuit of the second stage is earthed. The anode direct current 01' the second stage in It is oh the basis of this recognition according to the invention, in a cascade 'circuit, at least at rthe stages preceding the last stage, the point 9 is alone always connected to earth.

Examples of cascade .circuits according to the invention are shown in Figs. 2, 3 and Ain which the supply of the tubes is also indicated. Each of the cascade circuits shown comprises two stages in which those elements of the second stage which also occur in the first stage are designated by primed reference numerals.

In the circuit shown in Fig.

2 the points 0,!"

consequence flows across the cathode lead included in the input circuit so that the network ii, 82 which serves for the generation of the control grid bias has also to be included in this cathode lead. The bias set-up is impressed on the control grid 3 by means 01 a leak 30.

The output oscillatory circuit It or the second stage is connected in a high-frequency manner through a condenser 33 to the point).

in both stages are connected to earth in a hlghfrequency manner through condensers 23, 23' respectively which constitute a short-circuit for v the oscillations to be amplified.

The condensers 23 and 22' are connected in parallelwith resistances 24 and 24' across which the second stage should alone be earthed, the end of the oscillatory circuit ll which is remote from the anode I is not connected to earth but is connected, through a condenser '2', to that end 2i of the cathode lead included in the input circuit which-is remote from the cathode 2'. The anode alternating currentot the first stage consequently fiows from the anode 1' through the circuit l3, the condenser 2|, the inductances L1" and L2 and the condenser 22' to earth and thence through the condenser 22 and the incuit .of the second stage..- Since the point i of ductance L2 to the cathode 2. Since the anode current of the first stage is approximately in The accessible part of the anode lead and the output oscillatory circuit ii of the second stage are screened capacitatively relatively tovearth by means of a screen 24 connected through a conductor 35 to the point 0'.

Lastly, Fig. 4 shows a circuit in which the stages are entirely separated iromeach other and in which the output oscillator-y circuit l3 0! the firststage is coupled inductivelyto' the input.

oscillatory circuit l2 or the second stage. The two stages are .quite identical. In both stages the points 9, 8' are earthed in a high irequency manner and the output oscillatorycirc'iiits l3, II are connected to earth through respectively the condenser 38, I". 1

Instead 01' using an inductive coupling between the circuits II and I2 use may be made of a 'capacitative coupling, for example by means of a coupling condenser 22" arranged between the anode'l and the control grid 3', as shown in Fig. 4a.

The circuit shown in Fig. 4 may be enlarged with any number of stages.

What we claim is: 1. A cascade circuit comprising at least two high-frequency amplifying stages each 0! which comprises an amplifier valve -whose cathode is provided with at least two leads one 01' which, tor

the oscillations to be amplified, is included in the,

input circuit and another in the output circuit and in which a capacity is connected between the control grid and that end 0! the cathode lead remote from the cathode included in the output circuit and the latter cathode lead comprises an impedance of an inductive character, said capacity and impedance being proportioned in such manner that the input-damping is zero or negative, wherein at least at those stages which precede the last stage solely that end of the cathode lead included in the output circuit which is remote from the cathode is connected to earth in a highirequency manner.

2. A cascade circuit as claimed in claim 1, in which the output oscillatory circuit of the prenext following stage, and means for capacitively coupling the low potential end of said circuit to the cathode lead included in the input circuit of said next following stage at the end thereof 5, remote from the cathode.

ceding stage also constitutes the input oscillatory circuit of the next following stage, wherein the oscillatory circuit concerned is not earthed in a high-frequency manner but is connected to that end of the cathode lead of the said following stage included in the input circuit which is remote from the cathode.

3. A cascade circuit as claimed in claim 1, in which the output oscillatory circuit of the stage preceding the last stage also constitutes the input oscillatory circuit of the last stage, wherein the oscillatory circuit concerned is earthed in a highfrequency manner and the accessible part of the anode lead and the-output oscillatory circuit of the last stage are screened capacitatively relatively to earth. r

' 8. A multi-stage circuit according to the invention defined in claim 6'wherein the output circuit of a preceding stage also constitutes the input circuit of the next following stage, means for 10 capacitively coupling the high potential end of said circuit to the signal input grid of said next following stage, and means for connecting the low potential end of said circuit to ground.

9. A multi-stage circuit according to the invention defined in claim 6 wherein the output circuit of a preceding stage also constitutes the input circuit of the next following stage, means for capacitively coupling the high potential end of said circuit to the signal input grid of said next following stage, means for connecting the low 4. Acascade circuit as claimed'in claim 1, in

which use is made of completely'separated stages, wherein the output oscillatory circuit of a preceding stage is coupled inductively or capacitatively to the input oscillatory circuit of a following stage.

5. A multi-stage circuit for the amplification of high frequencies, each stage comprising an elec-' tron discharge tube provided with atleast a cathode, a signal input grid and an anode, the cathode having at least two leads connected thereto, input and output circuits connected respectively to the input. grid and to the anode, the grid return of the input circuit being connected to one of the cathode leads, the plate return of the output circuit being connected to I tron discharge tube provided with at least a cathode, a signal input grid and an anode, the cathode having at least two leads connected thereto, input and output circuitsconnected respectively to the input grid and to the anode, the rid return of the input circuit being connected to one of the cathode leads, the plate return of the output circuit being connected to the other cathode lead, a condenser connected between the signal input grid and that end of the cathode lead included in the output circuit which isremote from the cathode, and a condenser shunted by a resistance-connected in each stage between that end of the cathode output lead remote from the cathode and ground.

7. A multi-stage circuit in accordance with the invention defined in claim 6 wherein the output circuit of 'a preceding stage also constitutes the input circuit of the next following stage, means for capacitively coupling the high potential end of said circuit to the signal input grid of said potential end of said circuit to ground, and means -for capacitively screening theaccessible part of the anode lead and the output circuit of said next following stage. 1

coupled to the input circuit of the next following stage. v

11. A multi-stage circuit for theamplification of high frequencies, each stage comprising an electron discharge tube provided with at least a cathode, a signal input grid, a screen grid and an anode, the cathode having a pair of leads con-.

nected thereto, animpedance connected to one of the cathode leads, input and output circuits connected respectively tothe input grid and to the anode, the plate return of the output circuit being connected to the low potential end of the 40 impedance connected to said one cathode lead.

the grid return of the input circuit .being connected to the other cathode lead at the end remote from the cathode, capacitive means connected between the signal input grid and the low potential end of the cathode impedance included in the output circuit, and means in one of the stages preceding the last for grounding with respect to high frequencies that end of the oath ode impedance included in the output circuit which is remote from the cathode.

12. A multi-stage circuit for the amplification of high frequencies, each stage comprising an electron discharge tube provided with at least a cathode,.a signal input grid, a screen grid and an 1 anode. the cathode having a pair of leads connected thereto, an impedance connected to one of the cathode leads, input and output. circuits connected respectively tothe input grid and to the anode, the plate return of the output circuit .of at leastone stage being connected through a capacity and shunt resistance to the low potential end of the impedance connected to said one cathode lead, the grid return of the input circuit being connected to the other cathode lead at the end remote from the cathode, and capacitive means connected between the signal input grid and the low potential end of the cathode impedance included in'the output circuit.

MAmMImAAN JULIUS OTTO STRUTT.

ALDERT VAN nan ZIEL.

10. A multi-stage circuit i'n' accordance with. the'invention defined in claim 6 wherein the out: put circuit of a' preceding stage is inductively 

